Creutzfeldt-Jakob Disease or CJD is a fatal brain condition believed to be the human equivalent of Bovine Spongiform Encephalopathy (BSE), or mad cow disease. It was named after Creutzfeldt, the scientist who described the first case in 1920, and Jakob, the physician who diagnosed the next three.

Nobody knows precisely what causes the disease, which destroys brain tissue: the brains of sufferers become riddled with tiny holes and eventually resemble a sponge, hence the name spongiform, the class of diseases to which it belongs. Sheep and cows are the other main sufferers of spongiform diseases. Scrapie is the equivalent of mad cow disease in sheep.

The favoured theory is that CJD is not induced by traditional disease- causing agents, like bacteria and viruses, but by an undiscovered - indeed, theoretical - life form known as a prion. These agents are believed to consist entirely of protein and lack DNA and RNA, the "genetic blueprint" of an organism, which stores all the information needed to live and reproduce.

How will I know I've got it?

You won't, but your friends and family will. The first symptoms include poor concentration, depression and inexplicable feelings of fear, anxiety and aggression. Bodily co-ordination is then lost and trembling, writhing and uncontrolled jerking spasms set in. Patients then become mute, unresponsive and incontinent. They are also unable to eat. Sufferers then finally slip into a vegetative state.

Can I catch it from other people?

Probably not. It is not known to be easily transmissible between people; but at least 13 people in Britain treated for dwarfism have died from CJD after being given growth hormone extracted from human pituitary glands (thought to be from corpses that had been infected with the disease). The clearest evidence that spongiform diseases can pass between humans comes from the Fore tribe in Papua New Guinea, where Kuru, a disease very similar to CJD, was passed on through cannibalism. Some scientists see this as evidence that humans could catch CJD by eating infected meat.

What is mad cow disease?

This is the spongiform disease that afflicts, and now appears endemic, to cattle. The disease shows similar symptoms to CJD in humans. When the brains of cattle are dissected they also show the characteristic spongy texture seen in human sufferers and sheep afflicted by scrapie. Despite nine years' intensive effort scientists have been unable to find the agent that causes it. Nobody is sure whether the same agent is responsible for CJD, mad cow disease and scrapie. Most scientists claim the disease cannot pass to humans, but a few claim it can, and is rapidly doing so.

How did it spread to cows?

The cows became infected when they were fed the offal of sheep infected with scrapie. Intensive farming has now become highly integrated, with the waste materials from one part of the meat industry being used as the raw material for another. A major feature of the meat industry is the conversion of waste bones, meat and offal into animal food. In essence, vegetarian animals have been turned into meat-eaters.

After the scrapie-infected offal entered the food supply it seems to have developed the ability to infect cattle. The disease became magnified as the waste from slaughtered animals was rendered down and fed to fellow farm animals. At the same time it was entering the human food supply in cheap meat products like pies, sausages and burgers.

Mad cow disease was confirmed in 1986 after a spate of cases the previous year.

What did the Government and the meat industry do?

They repeatedly reassured the public that there was no cause for concern and that it could not possibly spread to humans. Meanwhile, cattle began to die by the hundred. After lots of bad publicity, the Government set up a committee to study mad cow disease under Sir Richard Southwood. Following its interim report, the practice of producing animal feed from rendered- down sheep and cattle slaughterhouse waste was banned (although the animal feed industry was, and still is, allowed to use recycled waste chickens and their manure). The Government also ordered the slaughter of all infected livestock.

The committee released its full recommendations in February 1989 and issued a stern warning to the intensive farming industry: "Considering BSE and how this new disease has arisen has led us to question the wisdom of some of the intensive practices of modern husbandry, because they risk exposing man to new zoonoses [diseases that humans can catch from animals]."

But it also reassured the public that beef was safe, stating that it was "most unlikely that BSE will have any implications for human health" - though adding that if it was wrong, "the implications would be extremely serious" and given the very long incubation in humans, "it may be a decade or more before complete reassurance can be given".

Was that the end of it?

No. The following year the Government decided to ban certain parts of the offal of cows and sheep from the human food chain. John Gummer, then Minister of Agriculture, also decided to reassure the British public by feeding Cordelia, his four-year- old daughter, a beefburger. The Department of Health established a surveillance unit in Edinburgh to spot any changes in the incidence of CJD in the human population that may be attributable to mad cow disease.

So is beef safe to eat?

Scientists are divided. Only a few mavericks suggest that there is any possible risk from eating steaks or other cuts of prime beef. The questions concern those parts of the animal where the disease is known to reside, such as the brain, spinal cord and thymus. Before 1989, large quantities of these offal entered the human food chain, and some may still be eaten. Most scientists still think that even eating these is unlikely to carry a risk. But the numbers warning that there could be a risk are growing. Last week's British Medical Journal carried seven articles on the subject. And Professor Bernard Tomlinson, a former neuropathologist and architect of the Government's health service reforms in London, said last week that he had stopped eating beef products like pies and burgers, which may still contain offal.

What is the evidence?

The number of deaths from CJD increased from 28 in 1985 to 55 last year. But the increase in cases is almost certainly a result of better diagnosis. The incidence of the CJD is fairly constant across Europe, where BSE is extremely rare, and corresponds to one death per million people per year. The Government maintains BSE is descended from scrapie and that since the British have lived with scrapie for over 200 years without a corresponding epidemic of CJD then the disease is unlikely to spread to humans. But the British as a rule haven't eaten the sheep parts known to harbour the disease. The disease appears to have been transmitted through infected feed to cats, ostriches, antelopes, pumas and cheetahs.

There is also the growing incidence of CJD in dairy farmers where it would first be expected to appear if it is transmissible to humans: four farmers and two young people have recently died. According to Dr Sheila Gore,of the Institute of Public Health in Cambridge, the "cases of Creutz- feldt-Jakob disease in farmers and young adults are more than happenstance. They signal an epidemiological alert to investigate intensively possible exposures - farm-related and dietary."

Why aren't scientists more certain?

Because the incubation period is very long and it is possible that many people who have eaten infected meat products will not show signs of the disease for years. Most scientists put the incubation period at between 15 and 40 years, though some say it is as short as 10. We'll have a better idea by the end of the decade.

After the cows stopped eating sheep and each other did BSE disappear ?

Unfortunately not. The incidence of BSE peaked, as predicted in 1993, at about 700 cases per week. But about 300 new cases per week are currently detected and another 600 cattle per week are estimated to be slaughtered with pre-clinical symptoms. All told, 155,000 BSE infected cattle have been slaughtered and burnt since the start of the cattle epidemic. The concern is that many of the current crop of victims were born after the ban on offal from cattle and sheep in their feed. Some scientists claim that this indicates BSE is now well entrenched in the cattle population and is being spread from mother to calf. The Ministry of Agriculture (MAFF) says the victims contracted BSE from infected material "leaking" into their food supply or were fed on old batches of cattle cake.

Are we still eating infected meat?

Probably. All beef offal was banned from the human food chain in 1989 but substantial quantities may be "leaking" into our food. Some estimates suggest the average meat-eater has consumed about 80 meals containing meat from infected animals. But the MAFF says all animal parts known to harbour the disease are removed after slaughter so do not enter the human food chain. But the ministry has tightened the rules on offal removal several times and slaughterhouse owners have consistently failed to implement the new health measures satisfactorily.

The State Veterinary Service made unannounced visits to 193 abattoirs in September and found failings in the handling of offal in 92. When inspectors visited 153 slaughterhouses in October they found failings in 52. Douglas Hogg, the Agriculture Minister, says most were of a "comparatively mild nature" but others were potentially serious. Some slaughterhouses failed to segregate and store banned offal properly,. In some cases small pieces of spinal cord were found attached to carcasses. The ministry said they were removed before meat left the slaughterhouses.

What do the conspiracy theorists say?

The Government has consistently protected the health of the farming and meat industry at the expense of the consumer's. The pounds 2bn dairy industry's lobbying power has the ear of the ministry - the consumer does not.